1. Field of the Invention
The present invention relates to DC-to-DC converters. More specifically, the present invention relates to resonant DC-to-DC converters.
2. Description of the Related Art
Sun et al. (U.S. Pat. No. 8,018,740) teaches operating an LLC resonant converter in a fixed-frequency and variable-duty-cycle operation mode during startup of the LLC resonant converter. The LLC resonant converter of Sun et al. switches to a variable-frequency and fixed-duty-cycle operation mode once startup is completed (e.g., when a predetermined voltage is reached at a load connected to an output of the LLC resonant converter). That is, during startup, the LLC resonant converter of Sun et al. operates in a pulse-width-modulated (PWM) mode, such that only the duty cycle is controlled and the frequency of the resonant converter remains fixed, and then switches to a pulse-frequency-modulation (PFM) mode after startup. Accordingly, precise control is needed in Sun et al. to determine a threshold for switching from the PWM mode to the PFM mode, which requires a complex control scheme provided by, for example, a digital controller. Further, conventional PFM controllers do not support controlling an LLC resonant converter as required by Sun et al.
The following problems can occur during startup of a resonant converter, for example, an LLC converter with PFM control.
The output voltage of the resonant converter can abruptly reach its maximum value within hundreds of micro-seconds of starting the resonant converter. Since a load of the resonant converter is typically capacitive, if the voltage of the resonant converter abruptly reaches its maximum value, a large inrush current can occur at the output of the resonant converter. Further, the voltage of the resonant converter abruptly reaching its maximum value can cause a voltage overshoot at the output of the resonant converter. The large inrush current and the voltage overshoot can damage both the resonant converter and the load.
Output voltage overshoot of the resonant converter can exceed a desired specification value.
A large inrush current in an output capacitor can decrease the life of components such as the output capacitor, switching elements of the resonant converter, a current transformer of the resonant converter, and a capacitor for detecting a resonant current. Further, a large inrush current can also damage the load connected to the output of the converter.
Over-current protection (OCP) can be activated by startup transients.
A large ripple voltage can appear at the output of the resonant converter due to a burst mode (i.e., the resonant converter quickly turning on and off for certain periods of time) at light load unless the switching frequency of the resonant converter can go very high. For example, if a normal operating frequency is about 200 kHz, at light load the operating frequency can reach about 800 kHz.